This invention is related to electrical switching and control circuits.
Electrical and electronic devices are frequently required to be operated and controlled remotely via suitable switching and control circuits. In the case of high power devices operating on relatively high voltages, such as devices intended for operation on the main electrical distribution network, the resulting control and switching currents can be significant reaching several hundred milliamperes. When currents of such magnitudes flow through long cables, there is generated a significant voltage across the cable whose magnitude is proportional to the specific impedance, or resistivity, of the cable. This, in turn, gives rise to energy wastage and increased operating costs. The voltage across the cable can be reduced by employing lower gauge (i.e. thicker) cables whose resistivity is correspondingly lower, but this results in the control and switching cables being bulky as well as expensive.
Published PCT application no. WO 95/31028, in the name of present inventor, discloses a detector for monitoring the integrity of a ground connection to an electrical appliance having live and neutral terminals for feeding current to the appliance from respective live and neutral feeders of an electrical supply having a ground point for connecting to the ground terminal of the appliance. The detector comprises a differential comparator circuit for comparing a voltage at the neutral connection with a voltage at the ground terminal of the appliance and producing a fault signal if a difference therebetween exceeds a predetermined threshold. A switching device is connected in at least one of the live and neutral connections so as to be opened by a relay operatively coupled to the detector and responsive to the fault signal produced thereby.
WO 97/36358 published on Oct. 2, 1997, in the name of the present applicant, discloses a specific application of such a detector for use with electrical appliances which are either ungrounded or whose ground connection is impaired. Thus, in accordance with WO 97/36358 there is provided a protection device for use in conjunction with an electrical appliance having an electrically conductive outer casing and which protects against the casing becoming xe2x80x9clivexe2x80x9d regardless of the state of a ground connection associated with an electrical supply to which the appliance is connected, the electrical appliance having live and neutral terminals for feeding current to the appliance from respective live and neutral feeders of the electrical supply, said device comprising:
a virtual ground connection which is electrically floating with respect to said ground connection of the electrical supply, said virtual ground connection for electrically coupling to the casing of the electrical appliance instead of the ground connection of the electrical supply, and
a ground impedance measuring circuit for measuring an impedance between either the live or neutral terminals and the virtual ground connection of the appliance and producing a fault signal if said impedance falls below a predetermined threshold.
In effect, the protection circuit disclosed in WO 97/36358 employs the differential comparator subject of WO 95/31028 to compare the voltages between live and neutral and a floating ground connection such that any discrepancy between the measured voltages is indicative of a ground fault. There are several major advantages of the use of a floating ground connection as opposed to a regular ground connection. First, there is no danger of a person effecting electrical contact with the floating ground connection becoming electrocuted since there is no return path through actual ground for the fault current. Second, by using appropriate resistors in the differential comparator circuit, the xe2x80x9cfaultxe2x80x9d current required to register an imbalance may be reduced to fractions of a nanoampere (i.e. less than 10xe2x88x929 ampere) as distinct from the milliamperes associated with conventional ground fault detector circuits. Yet another advantages is, of course, the lack of dependence on a reliable ground connection.
The use of a floating ground connection per se is known. Thus, EP 695 105 discloses a protection device for use with an appliance having a ground connection 33, which might be the metal casing of the electrical appliance, which is connected via a resistor R1 to a protection circuit comprising elements EC2, EC3, EC4 and OC1 so that if the leakage current flowing through R1 exceeds a certain threshold, this protection circuit provides a trigger to a relay coil RL1 so as to open the primary switch contacts SW1 and SW2. Thus, the protection circuit produces a fault signal if the ground impedance falls below a predetermined threshold.
Likewise, FR 2468430 discloses a protection device wherein, as in above-described EP 695 105, the principle of operation is that, in the event of a ground fault, there will be a leakage current flowing through the ground connection and the magnitude of this leakage current is employed in order to provide a trip signal for the main current breaker(s). In the case of a regular ground fault between either the live or neutral feeders and GND, the resulting ground leakage current which flows through the virtual ground connection effects adequate protection.
However, in the event of a short-circuit between the live and neutral connections, there will under normal circumstances be no ground leakage current and therefore the circuits described in EP 695 105 and FR 2468430 will afford no protection. This is a very serious drawback because short-circuit faults represent a significant risk of fire.
It is an object of the invention to provide a high sensitivity switching circuit for allowing remote switching of an electrical device via a pair of wires carrying negligible current whereby high gauge cables of minimal cross-sectional area may be employed without resulting in unacceptable ohmic losses.
In accordance with a broad aspect of the invention there is provided a switching circuit for use with an electrical appliance, the switching circuit comprising:
first and second normally open switch terminals,
a high sensitivity impedance measuring circuit coupled to the first and second switch terminals for measuring an impedance therebetween, said high sensitivity impedance measuring circuit producing at an output thereof a switching signal if said impedance is lower than a predetermined threshold of 500Mxcexa9;
the appliance being coupled to said output so as to be responsive to said switching signal.
The invention finds particular application for controlling and protecting electrical appliances operating from an a.c. electrical mains supply. For such applications, one of the switch terminals is connected to a virtual ground connection which is electrically floating with respect to a ground feeder of an electrical mains supply. In the event of electrical contact between the virtual ground connection and the other switch contact, an apparent ground fault is produced. The ground fault gives rise to a small current which is detected by the impedance measuring circuit so as to produce a switching signal which may then be used to operate a suitable relay for controlling the appliance.
A sufficiently strong fault signal may be generated upon contact between the virtual ground connection and ground by a human being having a body resistance typically in the order of several thousand ohms. By such means, human contact with one or both of the switch terminals is sufficient to register a xe2x80x9cground faultxe2x80x9d thus producing the required switching signal. If desired, one or both of the switch terminals may be connected to a metal touch plate such that momentary contact therewith by a human being results in the generation of the required switching signal.
Alternatively, both switch terminals can be mutually floating with respect to ground whilst exhibiting a very high contact impedance of several hundred Mxcexa9. Shorting the switch contacts, even by means of finger contact, reduces the impedance to below the threshold of the impedance measuring circuit, thereby produce the switching signal. In this context, it is to be understood that xe2x80x9cshortingxe2x80x9d means lowering the contact resistance between the two switch contacts to less than the predetermined threshold of 500Mxcexa9. The resulting current which then flows into the impedance measuring circuit may be as low as several nanoamperes.
According to a preferred embodiment of the invention, the impedance measuring circuit comprises a differential comparator circuit for comparing a fraction of the voltage between the live and neutral connections of the electrical supply with a voltage at the virtual ground connection. A fault signal is produced if a difference between the two voltages exceeds a predetermined threshold.